I cluster things around mealtimes when we are all at the table anyway. By lengthening that time by 15 to 20 minutes (often less for us right now), I can effortlessly get in "school" (I'm mostly an unschooler, anyway) without it bossing my whole day. If we miss a day or a meal here or there, that's okay, we've made up for it in some other way. Meaning, I don't stress about it if we happen to meet at the park for a picnic with someone else. Before (or after) you eat, let your ds read a page to everyone at breakfast. After lunch he can do a math worksheet, and after supper maybe a hand-writing one. Maybe at lunch before you eat, but after the kids have started, you can read a bit from a history book, or show a science movie (I put my laptop on the counter) or some clip from youtube. If you read something just before you eat, it prompts the conversation for the meal, and hey, you're sitting there anyway.
Dh reads to the kids even more at bedtime while I run around the house getting stuff done or goofing around on the internet. He reads the Bible, and from the Childhood of Famous Americans series. Dd is picking up quite a bit of history there, and from their conversations that follow. It bleeds over into things she brings up with me during the day.
Your ds is just a little younger than my dd; have you checked out the kindergarten standards for your state? Then you can have a direction to head with what you do, and you may find that he already knows a good deal of what he'd learn in school. With an August birthday, he could well be considered a kindergartener NEXT year, instead of this fall. I mention that to maybe take some of the pressure off of what you feel like you MUST do with him now. For example, in VA, the children are to learn to count to 10, forward and backward by the end of the year. You can teach that by singing it in the car on the way to wherever.
I find I feel far less scattered, even if I'm just doing bits here and there, if I have a list in my mind of where I'm headed. It's broken down into rough weekly increments, and it sort of guides what I do with my children. Ds is two, and I'm making sure he knows colors and shapes. So, I simply make it a point to show him all the ones I see, and to occasionally, casually, ask him what one is. After a while, when I'm confident he knows them, I'll move on to something else.
In short (sorry I've been so random), I build things into parts of our day that are routine anyway. When I'm actually working, I can't do something else, too, and the needs and demands of my chores are not routine. And my life is very unpredictable. But, we always eat, and the kids have to go to bed, and they have to take a bath. And we spend time in the car. So, I plan things for those times (and those are also times that I am already engaged with them and don't have other things nagging at me.)